1. Field of the Invention
The present invention relates to a color image forming method and a color image forming apparatus.
2. Description of the Related Art
For the purpose of imparting a high-grade sense to an image and increasing the durability thereof, an overcoat layer of varnish or the like has been conventionally provided on the surface of the image that appears on color pages, etc., of tickets, catalogues and magazines. Particularly, in commercial fields, varnish layers are formed on images printed in a great number by screen printing, etc. Although these images are in general high in percentage of image area, they can be prepared as beautiful images having a high-grade sense due to good compatibility of varnish with ink used in screen printing.
However, in recent years, there has been found an increasing demand for frequently changing and updating information to be printed. Therefore, a printing method such as screen printing which prepares an original text for printing is not economically feasible in many cases and so-called print on-demand has started to be used.
A recording method used in the above-described print on-demand usually includes an electrophotographic method and an inkjet method. The inkjet method is suitable for a small quantity of printed matter but unable to cope with a case that images are formed quickly and in a great quantity due to the long drying time of ink. Further, ink which has permeated into a recording medium such as paper will cause expansion or contraction of the paper to result in a slight change in thickness of the paper depending on a site of an image. It is, therefore, difficult to stack in an orderly manner a large quantity of ink recorded matter on which images are formed. As a result, the electrophotographic method using toner currently prevails. In the electrophotographic method, image information is exposed on a charged photoconductor to form a latent image, toner is used to develop the latent image, the thus obtained toner image is transferred to a recording medium such as paper and, thereafter, the transferred image is thermally fixed on the paper.
As a technology on the overcoat layer used in the electrophotographic method, there has been proposed, for example, an overcoat composition which is based on water, free of ammonia and low in static surface tension as an overcoat composition used for a to-be-printed matter on which fixing oil is coated (refer to Japanese Patent Application Laid-Open (JP-A) No. 2007-277547).
There have been also proposed a resin forming device which forms a silicone resin layer on a printing surface to protect the printing surface, giving waterproofing and imparting gloss, and an image forming apparatus which is provided with the resin forming device (refer to Japanese Patent Application Laid-Open (JP-A) No. 10-309876).
Further, there has been proposed a method for printing metal containers in which an electrophotographic method is used to efficiently print various types of printed matter in a small lot and a finish varnish layer is provided to protect a toner layer and impart gloss (refer to Japanese Patent (JP-B) No. 2522333).
The methods proposed above are all preferable in providing an overcoat layer on an electrophotographically formed image.
For an improvement in mold releasability, silicone oil has been coated in a great quantity on the surface of a fixing roller of a fixing unit used in an electrophotographic method. However, toner is greatly different in mold releasability between a site which has silicone oil on the fixing roller and a site which is free of silicone oil. Therefore, a site on which silicone oil is not coated will cause streaks which are different in gloss. Large-scale printing could increase costs accordingly, if the printing should fail. Further, where silicone oil adheres on a floor, the floor becomes quite slippery. Still further, since complete removal of silicone oil is difficult, full attention is required in supplementing the silicone oil and maintaining the fixing unit. And, this is troublesome for those involved in maintenance work.
In recent years, so-called oilless fixing has been carried out in which wax-containing toner is used to heat a toner image on fixing, allowing the wax contained in the toner to ooze out on the surface of the image, thereby securing mold releasability of the image from a fixing roller. In the oilless fixing, where wax is present in a greater quantity between the image on fixing and the fixing roller, the mold releasability of the image from the fixing roller is further improved. For this reason, wax is added to toner to the extent possible, and such wax that easily melts at a low temperature is used to appropriately adjust fixing conditions (pressure of the fixing roller, fixing temperature and fixing time) which facilitates melting of the wax.
However, in an attempt to provide an overcoat layer on an image on which the above-described oilless fixing has been carried out, wax on a fixed image repels an overcoat composition. Thus, there are problems that the overcoat layer becomes quite thin in thickness at a site great in image area, the cured overcoat layer is not firmly attached on an oilless fixed image and scratching or bending of the surface results in detachment of the overcoat layer.
Further, a color image is formed by overlapping respective yellow, magenta, cyan and black color toners on a recording medium. Therefore, the color image is greater in quantity of toner adhered than a single-color black-and-white image and also greater in content of wax. As a result, the color image is further decreased in attachment property of the overcoat layer than the black-and-white image, which makes the above problems more apparent.